Superconducting magnet systems are used in a variety of contexts, including nuclear magnetic resonance (NMR) analysis, and magnetic resonance imaging (MRI). To realize superconductivity, a magnet is maintained in a cryogenic environment at a temperature near absolute zero. Typically, the magnet system includes one or more electrically conductive coils operating as one or more magnets and which are disposed in a cryostat and cooled by a cryogenic fluid such as liquid helium to maintain superconductivity.
Some superconducting magnet systems have multiple superconducting coils or coil sections at different axial locations, as well as with different radii. For example, in some cases a superconducting magnet system for an MRI apparatus may include a main superconducting coil (sometimes called a field coil) having a plurality of main coil sections (or field coil sections) at different axial positions which have a first radius, together with a superconducting shield coil having a plurality of shield coil sections at different axial positions and which have a second radius which is greater than the first radius of the field coil.
When energized, the superconducting coils or coil sections, whose diameters for example may be on the order of one or two meters, impose large magnetic forces between them as well as large internal magnetic stresses. Additionally, when the superconducting coils are energized, the coil sections expand radially by a small amount (e.g., 1 mm or more), and this amount of radial expansion may vary according to the diameter of the coil. However, in an MRI apparatus the superconducting coils must be positioned relative to each other to within fractions of a millimeter in order to produce magnetic fields suitable for imaging. So the radial expansion of the coils upon energization presents a challenge.
Furthermore, the superconducting coils must also be kept at very low temperatures, for example disposed within a cryostat, in order to remain superconducting. Whatever structure supports the superconducting coils must locate the superconducting coils to within specified tolerances under large magnetic loads as discussed above, while not creating any energy release into the coils which could heat them or the surrounding area and thereby impact their superconducting performance.
The interface between the superconducting coils and the support structure or device can present a number of challenges. Restraining the superconducting coils and positioning the coils can create large shear stresses at the interface between the superconducting coils and the support structure. Meanwhile, large axial forces combined with large hoop forces may cause a release of energy at the interface(s) between the coil(s) and the support structure though friction or cracked bonds, and this energy may quench superconducting coils.
For MRI, the superconducting coils often use wire made with small filaments of NbTi which carries the high currents without resistance. The filaments are embedded in a copper matrix which allows current to be evenly distributed and provides stability. Often additional copper is used in the wire to both further stabilize the coils from external energy releases and to lower stress levels at the interface between the coils and the support structure. However, adding additional copper to the superconducting coils makes them less efficient magnetically, increases their thermal capacity (which takes longer to cool them down), and increases their cost.
Accordingly, it would be desirable to provide an apparatus, such as an MRI machine, which includes a support structure for a system of superconducting coils. It would further be desirable to provide such a support structure which can maintain the positions of the superconducting coils relative to each other to within tight tolerances when energized. It would still further be desirable to provide such a support structure which can accomplish these objectives without producing a large amount of frictional energy and associated heat dissipation.
In one aspect of the present invention, an apparatus comprises: at least a first electrically conductive coil having at least first and second coil sections which are separated and spaced apart from each other; and a support structure disposed to support the first and second coil sections. The support structure is configured to maintain relative axial positions of the first and second coil sections to be fixed when the first electrically conductive coil is energized and de-energized, and to allow each of the first and second coil sections to expand radially when energized.
In some embodiments, the apparatus further can further comprise: at least a second electrically conductive coil, wherein the first electrically conductive coil can be a field coil and the second electrically conductive coil can be a shield coil, wherein the shield coil can have at least first and second shield coil sections which are separated and spaced apart from each other wherein the shield coil can have a diameter which is greater than a diameter of the field coil, and wherein an axis of the shield coil can pass through a circumference defined by the field coil. The support structure can be configured to maintain relative axial positions of the first and second shield coil sections to be fixed when the first electrically conductive coil is energized and de-energized, and allow each of the first and second shield coil sections to expand radially when energized.
In some versions of these embodiments, the support structure can comprise: a first support element which can have a first portion which is disposed at a first site on the first coil section, and a second portion which is disposed at a first site on the second coil section, wherein the first support element can axially, radially, and rotationally fix the first site on the first coil section with respect to the first site on the second coil section; a second support element which can have a first portion which is disposed at a second site on the first coil section, and a second portion which is disposed at a second site on the second coil section, wherein the second support element can axially and rotationally fix the second site on the first coil section with respect to the second site on the second coil section, and allow radial movement of the first and second coil sections; a third support element which can have a first portion which is disposed at a third site on the first coil section, and a second portion which is disposed at a third site on the second coil section, wherein the third support element can axially fix the third site on the first coil section with respect to the third site on the second coil section, and allow radial and rotational movement at the third sites of the first and second coil sections; and a fourth support element which can have a first portion which is disposed at a fourth site on the first coil section, and a second portion which is disposed at a fourth site on the second coil section, wherein the fourth support element can axially fix the fourth site on the first coil section with respect to the fourth site on the second coil section, and allow radial and rotational movement at the fourth sites of the first and second coil sections.
In some versions of these embodiments, the support structure can further comprise an electrically insulating support ring having an axis which extends in parallel to an axis of the first electrically conductive coil, wherein the electrically insulating support ring can be fixedly attached to the first support element, and wherein each of the third and fourth support elements can have a slot disposed therein, wherein a first portion of the electrically insulating support ring can be disposed in the slot in the third support element, and a second portion of the electrically insulating support ring can be disposed in the slot in the fourth support element.
In some versions of these embodiments, the electrically insulating support ring can include at least one rotational restraint, wherein the second support element can have a slot disposed therein, wherein a third portion of the support ring can be disposed in the slot in the second support element, and wherein the second support element can be rotationally fixed by the at least one rotational restraint.
In some versions of these embodiments, the apparatus can further comprise at least first and second protrusions extending from the electrically insulating support ring, wherein the second support element can have a slot disposed therein, wherein a third portion of the support ring can be disposed in the slot in the second support element, and wherein the second support element can be rotationally fixed between the first and second protrusions.
In some versions of these embodiments, the apparatus can further comprise at least a second electrically conductive coil, wherein the first electrically conductive coil can be a field coil and the second electrically conductive coil can be a shield coil, wherein the shield coil can have at least first and second shield coil sections which are separated and spaced apart from each other, wherein the shield coil can have a diameter which is greater than a diameter of the field coil, wherein an axis of the shield coil can pass through a circumference defined by the field coil, and wherein the support structure can be configured to maintain relative axial positions of the first and second shield coil sections to be fixed when the first electrically conductive coil is energized and de-energized, and can allow each of the first and second shield coil sections to expand radially when energized.
In some versions of these embodiments, the support structure can be configured to cause the axis of the field coil and the axis of the shield coil to be nonaligned when at least one of the field coil and the shield coil is de-energized, and to cause the axis of the field coil and the axis of the shield coil to be aligned coaxially when the field coil and the shield coil are both energized.
In some versions of these embodiments, the first support element can have a third portion which is disposed at a first site on the first shield coil section, and a fourth portion which is disposed at a first site on the second shield coil section, wherein the first support element can axially, radially, and rotationally fix the first site on the first shield coil section with respect to the first site on the second field coil section, and the support structure can further comprise: a fifth support element which has a first portion which is disposed at a second site on the first shield coil section, and a second portion which is disposed at a second site on the second shield coil section, wherein the fifth support element can axially and rotationally fix the second site on the first shield coil section with respect to the second site on the second shield coil section, and allow radial movement of the first and second shield coil sections; a sixth support element which has a first portion which is disposed at a third site on the first shield coil section, and a second portion which is disposed at a third site on the second shield coil section, wherein the sixth support element can axially fix the third site on the first shield coil section with respect to the third site on the second shield coil section, and allow radial and rotational movement of the first and second shield coil sections; and a seventh support element which has a first portion which is disposed at a fourth site on the first shield coil section, wherein the seventh support element can axially fix the fourth site on the first shield coil section with respect to the fourth site on the second shield coil section, and allow radial and rotational movement of the first and second shield coil sections.
In some versions of these embodiments, each of the sixth and seventh support elements can have a slot disposed therein, wherein a fourth portion of the support ring can be disposed in the slot in the sixth support element, and a fifth portion of the support ring can be disposed in the slot in the seventh support element.
In some versions of these embodiments, the apparatus can further comprise at least third and fourth protrusions extending from the electrically insulating support ring, wherein the fifth support element can have a slot disposed therein, wherein a sixth portion of the support ring can be disposed in the slot in the fifth support element, and wherein the fifth support element can be rotationally fixed between the third and fourth protrusions.
In some embodiments, the first electrically conductive coil can comprise copper and an electrically superconductive material.
In some embodiments, the support structure can comprise: at least one support ring having an axis which extends in parallel to an axis of the first electrically conductive coil; a plurality of support beams connected to the support ring; a plurality of first support elements each connected to the first coil section; and a plurality of second support elements each connected to the second coil section; a plurality of hinge member pairs, each hinge member pair connecting one of the first and second support elements to one of the support beams, wherein the first and second support elements can be configured to inhibit rotational and axial movement of the first and second coil sections with respect to each other, and wherein the hinge member pairs can allow radial movement of the first and second coil sections.
In some versions of these embodiments, the apparatus can comprise at least a second electrically conductive coil, wherein the first electrically conductive coil can be a field coil and the second electrically conductive coil can be a shield coil, wherein the shield coil can have at least first and second shield coil sections which are separated and spaced apart from each other, wherein the shield coil can have a diameter which is greater than a diameter of the field coil, wherein an axis of the shield coil can pass through a circumference defined by the field coil, and wherein the support structure can be configured to maintain relative axial positions of the first and second shield coil sections to be fixed when the first electrically conductive coil is energized and de-energized, and allow each of the first and second shield coil sections to expand radially when energized.
In some versions of these embodiments, the support structure can further comprise: a plurality of third support elements each connected to the first shield coil section; a plurality of fourth support elements each connected to the second shield coil section; and a plurality of second hinge member pairs, each second hinge member pair connecting one of the third and fourth support elements to one of the support beams, wherein the third and fourth support elements can be configured to inhibit rotational and axial movement of the first and second shield coil sections with respect to each other, and wherein the second hinge member pairs can allow radial movement of the first and second shield coil sections.
In another aspect of the present invention a method is provided for supporting at least a first electrically conductive coil having at least first and second coil sections which are separated and spaced apart from each other. The method comprises: maintaining relative axial positions of the first and second coil sections to be fixed when the first electrically conductive coil is energized and de-energized; and allowing each of the first and second coil sections to expand radially when energized.
In some embodiments, the method can further comprise: axially, radially, and rotationally fixing a first site on the first coil section with respect to a first site on the second coil section; axially and rotationally fixing a second site on the first coil section with respect to a second site on the second coil section, while allowing radial movement of the first coil section at the second site on the first coil section and allowing radial movement of the second coil at the second site on the second coil section; axially fixing a third site on the first coil section with respect to a third site on the second coil section, while allowing radial and rotational movement of the first coil section at the third site on the first coil section and allowing radial and rotational movement of the second coil section at the third site on the second coil section; and axially fixing a fourth site on the first coil section with respect to a fourth site on the second coil section, while allowing radial and rotational movement of the first coil section at the fourth site on the first coil section and allowing radial and rotational movement of the second coil section at the fourth site on the second coil section.
In some versions of these embodiments, the method can further comprise supporting at least a second electrically conductive coil, wherein the first electrically conductive coil can be a field coil and the second electrically conductive coil can be a shield coil, wherein the shield coil can have at least first and second shield coil sections which are separated and spaced apart from each other, wherein the shield coil can have a diameter which is greater than a diameter of the field coil, wherein an axis of the shield coil can pass through a circumference defined by the field coil. The method can further comprising: maintaining relative axial positions of the first and second shield coil sections to be fixed when the first electrically conductive coil is energized and de-energized, and allowing each of the first and second shield coil sections to expand radially when energized.
In some versions of these embodiments, the method can further comprise the axis of the field coil and an axis of the shield coil being nonaligned when at least one of the field coil and the shield coil is de-energized, and the axis of the field coil and the axis of the shield coil being aligned coaxially when the field coil and the shield coil are both energized.
In some versions of these embodiments, the method can further comprise: axially, radially, and rotationally fixing a first site on the first shield coil section with respect to a first site on the second shield coil section; axially and rotationally fixing a second site on the first shield coil section with respect to a second site on the second shield coil section, while allowing radial movement of the first shield coil section at the second site on the first shield coil section and allowing radial movement of the second shield coil at the second site on the second shield coil section; axially fixing a third site on the first shield coil section with respect to a third site on the second shield coil section, while allowing radial and rotational movement of the first shield coil section at the third site on the first shield coil section and allowing radial and rotational movement of the second shield coil section at the third site on the second shield coil section; and axially fixing a fourth site on the first shield coil section with respect to a fourth site on the second shield coil section, while allowing radial and rotational movement of the first shield coil section at the fourth site on the first shield coil section and allowing radial and rotational movement of the second shield coil section at the fourth site on the second shield coil section.
In some embodiments, the method can further comprise allowing each of the first and second coil sections to expand radially when energized via a plurality of hinged support elements each connected to one of the first coil section and the second coil section.
In some versions of these embodiments, the method can further comprise: supporting at least a second electrically conductive coil, wherein the first electrically conductive coil can be a field coil and the second electrically conductive coil can be a shield coil, wherein the shield coil can have at least first and second shield coil sections which are separated and spaced apart from each other, wherein the shield coil can have a diameter which is greater than a diameter of the field coil, wherein an axis of the shield coil can pass through a circumference defined by the field coil. The method can further comprise: maintaining relative axial positions of the first and second shield coil sections to be fixed when the first electrically conductive coil is energized and de-energized, and allowing each of the first and second shield coil sections to expand radially when energized via a plurality of additional hinged support elements each connected to one of the first shield coil section and the second shield coil section.
In yet another aspect of the invention, an apparatus comprises: an electrically conductive field coil; an electrically conductive shield coil, wherein the shield coil has a diameter which is greater than a diameter of the field coil, and wherein an axis of the shield coil passes through a circumference defined by the field coil; and a support structure disposed to support the field coil and the shield coil. The support structure is configured to cause an axis of the field coil and the axis of the shield coil to be nonaligned when at least one of the field coil and the shield coil is de-energized, and to cause the axis of the field coil and the axis of the shield coil to be aligned coaxially when the field coil and the shield coil are both energized.
In some embodiments, the field coil can have at least first and second field coil sections which are separated and spaced apart from each other, and the field coil can have at least first and second field coil sections which are separated and spaced apart from each other.
In some versions of these embodiments, the support structure can be configured to maintain relative axial positions of the first and second field coil sections to be fixed when the electrically conductive field coil is energized and de-energized, and to allow each of the first and second field coil sections to expand radially when energized, and the support structure can be further configured to maintain relative axial positions of the first and second shied coil sections to be fixed when the electrically conductive shield coil is energized and de-energized, and to allow each of the first and second shield coil sections to expand radially when energized.
In some versions of these embodiments, the support structure can further comprise: an electrically insulating support ring having an axis which extends in parallel to the axis of the field coil and the axis of the shield coil; and a plurality of support elements operatively engaged with the field coil, the shield coil, and the electrically insulating support ring so as to fix axial positions of the field coil and the shield coil with respect to each other, while permitting radial expansion of the field coil and radial expansion of the shield coil, wherein an amount of radial expansion of the field coil is different from an amount of radial expansion of the shield coil.